As is known in the art, automotive sensors may generally provide engine control, stability or suspension control, and outer situation monitoring. Among these, outer situation monitoring may be provided from an automobile radar disposed on a vehicle. Such radars are generally referred to as vehicle collision avoidance systems. Such systems aid a driver for example by warning the driver of an incoming object. As a warning sensor, or so-called anti-collision radar, the radar should be able to detect objects at a relatively long distance in contrast to a short range sensor which may be useful for parking, for example. Such anti-collision radars may be used on a variety of vehicles including but not limited to cars, trucks, and buses.
To provide a radar having a relatively long range detection capability, signals in the infrared and millimeter wave frequency ranges may be used. Millimeter wave frequency signals are generally preferred since they are relatively unattenuated by adverse weather conditions such as fog, rain, or snow. Furthermore, RF signals in the millimeter wave frequency range are substantially unaffected by road dust which may adhere to an antenna.
The presently assigned frequency ranges for automotive anti-collision radars vary from country to country. For example, in Europe the present frequency range is about 77 GHz, in Japan 60 GHz, and in the United States 24 GHz. One problem with anti-collision radar antennas used today, however, is that they fail to have steerable antenna beams.
One solution to this problem is to provide an anti-collision system having an electronically controlled phased array antenna. Electronically controlled phased array antennas typically have relatively high development and manufacturing costs, are susceptible to damage, and can be relatively difficult, expensive and time consuming to repair. Thus, it would be desirable to provide a relatively simple, robust antenna for some applications in which phased arrays are used.
One approach is to provide an antenna having discrete ferrite phase shifting circuits coupled thereto. Ferrite materials, however, have not generally been used in antenna apertures as either a radiator or a scatterer to provide electronic control of the antenna.
As is also known, there has been a trend to fabricate antennas which operate at relatively high RF frequencies including those frequencies in the so-called millimeter wave (MMW) or Ka-band frequency range. Conventional MMW beam techniques which provide an antenna having beam steering capabilities involve the use of periodic structures or scatterers which are provided as conductive gratings, diodes, or varactors. Alternatively, electronic steering may be provided by using delay lines having relatively small delay times (e.g. less than 1 nanosecond) at MMW frequencies.
Conventional two dimensional electronic steerable MMW antennas are generally constructed using delay lines which control the phases of each individual element in an antenna array. However, at MMW frequencies the wavelengths are small and the adjustment of the amount of phase shifts for an array becomes a formidable task.
It would thus be desirable to provide an array of antennas which may be incorporated into an anti-collision radar system and which has a steerable antenna beam. It would also be desirable to provide an alternative way to construct two dimensional steerable antennas for use in an anti-collision radar.